Hmm... perpendicular recording... Don't have opinion on this one, sorry. These are too new to get any reliability record, but on the other hand, Seagate is offering 5 years warranty on their drives, so it looks interesting.
My personal choice is WD (I know.. outdated technology and such) and Samsung. Had very bad experience with Maxtor and DeathStar.

Can you stretch your budget to WD Raptor 150GB? These are fastest drives on the market unless you want ot go for RAID0.WD1500AD Raptor X-Tends Performance Lead.
The article above is about X-model, but unless you definitely want to have window in your hard drive (why?) then go for cheaper non-X model.

Hmm... perpendicular recording... Don't have opinion on this one, sorry. These are too new to get any reliability record, but on the other hand, Seagate is offering 5 years warranty on their drives, so it looks interesting.

These drives are new, and a nice choice. Same price as for the normal drives, but faster.

Are you sure you don't want to hang on until more AM2 boards come out? A 2GHz AM2 Sempron on a good board is going to be a very nice o/c, and even if you definitely want an X2 the prices will drop once the new boards start to become available end of May/through June.

To be honest, I think the price drop has already hit on the existing x2s, some AM2 boards and processors are available now, and I saw the exising ones drop by around 20 quid a couple of weeks ago. Please tell me if you think I'm wrong about this, coz I'm prepared to wait a little longer ;)

Personally, I would hold out for a little longer. But that's me - I know it's hard to not buy once you've decided to. :)

However, do consider buying a top-end motherboard, for example the Asus A8R32 which has very good reviews and allows a huge overclock, and saving some money on your processor. For example, the X2-3800 and Opteron 170 will give very good results.

I can never justify the extra expense of going for the really hot products to squeeze a little extra which they weren't designed to produce. (Just my take on it!). The mobo you suggested is another £60 which more than pays for my 4400 (over the opty).

...to squeeze a little extra which they weren't designed to produce. (Just my take on it!).

Nah na, they're not priced to produce... :) Take my AMD Athlon XP 2500+ Barton from a few years back. Ran on a 333MHz bus. Exact same chip as the 3200+ which was about £200 more and ran on a 400MHz bus. So, on the motherboard upped the FSB from 166MHz to 200MHz (DDR, so double-pumped to get 333 and 400 respectively), and I have a 3200+ for about £100 as opposed to £300. The same is true for most AMD chips. Another example, the Athlon 64 3200+ running at 2GHz can be fairly easily o/ced to 2.6GHz (or even 2.7) giving the same level of performance as the 4000+ (even FX55) for far less cost. Intel have also released their 805, which can apparently be o/ced up to 4GHz per core... which is one hell of an overclock from 2.66GHz per core. This is why I reckon the AM2 Semprons will be fairly impressive, e.g. the 3500+ running at 2GHz.

If you have the money for a retail chip, that's great. It does mean you won't void the warranty. :)

Just a quick word on the case - really good choice. I have a Sonata first edition and it really is a well built and good looking case. If you are looking for more noise reduction there are some extra pieces of sound padding you can buy for the Sonata.

I think overclocking is definitely the way to go. I built a new pc last month, and after weeks of researching I bought a Opteron 165 (stock speed of 1.8GHZ) and with nothing more than a £25 Zalman airoflower CPU fan I have overclocked it to 2.6GHZ. This isn't dangerous and you won't damage the CPU unless you increase the core voltage significantly. I achieved 2.6GHZ without having to raise the core voltage at all, so the lifespan of my processor won't be affected at all.
Opteron processors are designed as heavy usage server processors, so they are much better than the AMD X2 series for overclocking. When the chips are produced the Opterons go through far tougher tests than X2's to check their quality.

I went for the Antec Titan case, it's a real monster and weighs a ton, but for £110 it comes with a Antec Trupower 550Watt PSU that I would highly reccommend it.